1. Field of the Invention
The present invention relates to power converters. More specifically, the present invention relates to start-up circuits that provide start-up voltage to a control system of a power converter during a power-up process when the output voltage rises from zero to a nominal level.
2. Description of the Related Art
According to one conventional start-up technique, an energy storage capacitor placed across the input terminals of a control circuit is charged up to the start-up level through a resistor connected to the input voltage. One drawback of this conventional technique is that the capacitance value and the physical size of the energy storage capacitor must both be relatively large to provide sufficient energy for the startup process to commence. An electrolytic or other polarized type capacitor is typically used for this purpose. Another drawback of this conventional technique is that the current charging the energy storage capacitor, and consequently the start-up time, depends on the input voltage, which limits this conventional technique to being usable only in power converters with relatively narrow input voltage ranges. Moreover, power losses in the charging resistor under steady state conditions of this conventional technique will increase monotonically with the input voltage.
A modification of the above-described start-up technique based on an energy storage capacitor is shown in FIG. 4 of U.S. Pat. No. 5,812,385. The circuit 100 in FIG. 1 is similar to FIG. 4 of U.S. Pat. No. 5,812,385. The start-up circuit 100 in FIG. 1 operates in the following manner. DC voltage is applied to terminals Vin+ and Vin− and charges input filter capacitor 101. Initially, transistor 103 is turned ON by pull-up resistor 106 and charges energy storage capacitor 105 through starting resistor 102 and diode 104. When voltage across energy storage capacitor 105 reaches a specific level, control circuit 112 is activated. The activated control circuit 112 supplies a CONTROL signal to converter 113 that starts the converter 113, which in turn supplies a control circuit bias voltage though diode 111. This control circuit bias voltage reverse biases diode 104 and supplies current to the control circuit 112. Control circuit 112 then supplies a VREF signal to transistor 107 through base current limiting resistor 109. The VREF signal turns the transistor 107 ON, which in turn turns transistor 103 OFF, saving power from being dissipated through resistor 102. During the power saving mode of control circuit 112, the CONTROL signal supplied to the converter 113 turns OFF, which causes the voltage supplied through diode 111 to capacitor 105 to begin to decay. When the voltage across capacitor 105 drops to a predetermined level, the control circuit 112 turns OFF and the VREF signal drops to zero, turning transistor 107 OFF and transistor 103 ON to restore power supplied from the DC input. This operation occurs in a cycle-by-cycle basis during the power saving mode of operation.
However, this conventional technique does not fully eliminate start-up power dissipation. Charging resistor 102 does not dissipate power under a steady state condition because, when transistor 103 is OFF, resistor 106 is still connected across the input DC voltage of capacitor 101 though closed transistor 107. If the input voltage range is relatively narrow, power dissipation in resistor 106 will be relatively low and can be discounted. However, if the input DC voltage has a wide range, then power dissipation in resistor 106 cannot be neglected because power dissipation increases directly proportional to the square of the input DC voltage. Assuming, for example, a 10:1 input voltage range and a 100 mW dissipation in resistor 106 at low input voltage, then the power dissipated in resistor 106 at high input voltage will be 0.1 W*(10)2=10 W. This is a significant change in the power dissipated by resistor 106.
Accordingly, the above conventional technique is undesirable because it requires the capacitor 105 to be relatively expensive, large value, and physical size, because it does not have a fixed start-up time, and the power dissipation losses under steady state conditions limit start-up circuits based on energy storage capacitors to only those that have a relatively narrow input voltage range.
Another conventional start-up technique is illustrated in FIG. 2. A start-up circuit 200 includes a start-up transistor 201, first diode 202, resistor 203, zener diode 204, filter capacitor 205, second diode 206, and power converter 207 with control circuit 208 and with auxiliary output terminals 209, 210.
The start-up circuit 200 in FIG. 2 operates in the following manner. After the input voltage Vin is applied to terminals Vin+, Vin−, resistor 203 supplies current to zener diode 204 and to the base of transistor 201. Transistor 201 supplies a start-up voltage at the input of the control circuit 208 and across the filter capacitor 205 equal to the zener voltage Vz of the zener diode 204 minus the combined voltage drops of transistor 201 and first diode 202. The start-up voltage reverse biases the second diode 206 and is supplied to the control circuit 208, which initiates the start-up process of the power converter 207. During this start-up process, the output voltage supplied by power converter 207 to the LOAD and the auxiliary voltages supplied by power converter 207 to auxiliary output terminals 209, 210 rise to their nominal levels. Because the start-up current for the control circuitry 208 is supplied by the transistor 201 that is controlled by the fixed zener voltage Vz of the zener diode 204, the control circuitry 208 functions independently of the input voltage. Thus, the start-up time is independent of the input voltage Vin.
Another significant difference is that filter capacitor 205 functions as a filter capacitor rather than an energy storage capacitor as the energy storage capacitor 105 depicted in FIG. 1. It should be noted that this capacitor 205 is not essential to circuit operation and is solely used for noise reduction. Because the filter capacitor 205 has a different function than energy storage capacitor 105, the value and size of filter capacitor 205 can be significantly smaller than energy storage capacitor 105. Additionally, capacitor 205 also can be a multi-layer ceramic capacitor, which provides savings in product cost in comparison to the cost of the energy storage capacitor 105.
When the auxiliary voltage across auxiliary output terminals 209, 210 exceeds the start-up voltage at the input of the control circuit 208, second diode 206 is forward biased, first diode 202 is reversed biased, transistor 201 switches to the OFF state, and auxiliary power from output terminals 209, 210 is supplied to the control circuit 208.
The resistance value R of resistor 203 is selected in accordance with the following equation:R=(Vin min−Vz)/Imin  (1)where Vin min is the minimum input voltage and Imin is the minimum current in the resistor R needed both to activate zener diode 204 and to supply base current to the transistor 201. At input voltages greater than Vin min, the current I through the resistor 203 increases according to the formula:I=(Vin−Vz)/R  (2)
Power dissipation P in the resistor 203 at high input voltage Vin max is defined by the following formula, which is based on the equations (1) and (2):P=Imin*(Vin max−Vz)2/(Vin min−Vz)  (3)
If the input voltage range is narrow, the power dissipation P in the resistor 203 is not significant and can practically be neglected. However, if the input voltage range is wide, the power dissipation P requires a physically larger resistor size, thus causing overall efficiency deterioration and an increasing no-load current. For example, consider a power converter with output power of Po=100 W and efficiency η=90% at minimum input voltage Vin min=16 V, with Vz=12 V and Imin=1.5 mA. Power dissipation P in the resistor 203 calculated according to the formula (3) and efficiency η for Vin max levels of 36 V, 75 V, and 150 V are shown in Table 1.
TABLE 1Vin max (V)3675150P (W)0.221.497.14η (%)89.8388.8284.57
The above example demonstrates that the conventional start-up circuit 200 of FIG. 2 is efficient enough in a relatively narrow input voltage range and is not efficient in a wide input voltage range. In the above example, at high input voltage Vin=150 V, the efficiency drops by (90%-84.57%)=5.43%, and the rated power of the resistor 203 must be increased to tolerate power dissipation of 7.14 W.
Thus, there is a need in the power conversion field for a more efficient start-up circuit for power converters with a wide input voltage range.